It's not showing that at all -- I chose those cans because they're obviously galvanized on the inside. They certainly soldered up like there's nothing in between the solder and the can. All of the other tin cans that we go through are obviously coated with some sort of plastic, which I'd have to clean off, and which may not leave me with a nice surface to which to solder.
I'm mostly concerned with leaving acid residue inside my nice model airplane motors after running them with a tank made with this flux.